GHC 6.12.1 panic

GHC 6.12.1 reports the following when compiling the attached module:

$ ghc --make Trampoline.hs -O[1 of 1] Compiling Control.Concurrent.SCC.Trampoline ( Trampoline.hs, Trampoline.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.1 for x86_64-unknown-linux):
	expectJust chooseExternalIds: ds_d1HQ

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

So I'm dutifully reporting it. The module in question could probably be easily trimmed down, but I'm too sleepy at the moment. Let me know if you need that.

Edited Mar 09, 2019 by Simon Marlow

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GHC 6.12.1 panic